To measure the degree of nasality is important in the fields of otorhinolaryngology, dentistry and oral maxillo-facial surgery. For instance, a person with a cleft palate produces considerable nasal sounds compared with a person with a normal palate. If the degree of nasality is clearly indicated in terms of numerical figures, the degree of cleft palate can be objectively evaluated.
A nasality indicator has been proposed to numerically indicate the degree of nasality. This conventional nasality indicator will be described below with reference to FIG. 1. A vibration detector 1 is placed at a desired position on the face to detect vibrations that are produced in the nasal wall during the utterance of voice sounds that pass through the nasal cavity. At the same time a microphone 2 is disposed in front of the lips at a predetermined distance to detect the sound waves produced during the utterance. Outputs of the detector and microphone are then compared in accordance with the following relation (1) to find the degree of nasality. ##EQU1##
As shown in FIG. 1, the individual outputs of the detector 1 and microphone 2 are amplified through amplifiers 3, 4, detected though detector/rectifier circuits 5, 6, applied to a comparison circuit 7, which determines the ratio V/S of the two outputs (V denotes an output level of the vibration detector 1 and S denotes an output level of the microphone 2), and the output of the V/S circuit 7 is indicated on a reading instrument 8.
FIG. 2 is a sectional view of the face which illustrates the paths for transmitting voiced sounds. The vibration detector 1 is installed on an upper side portion of the nose by suitable means, such as a double-sided adhesive tape or the like. The signals picked up by the vibration detector 1 consist of a signal b, which vibrates walls of a vocal tract when the phonated voice passes through the vocal tract and which is affected by resonance in the nasal cavity, and signals a, c, which are not affected by resonance in the nasal cavity. Here, the signal a, during phonation, is transmitted to the vibration detector 1 via an oral cavity, the signal b is transmitted to the vibration detector 1 via a nasal cavity, and the signal c is transmitted to the vibration detector 1 via paths other than via the oral and nasal cavities. The two arrows indicate voice paths.
The degree of nasality is given by: EQU V/S=(V.sub.1 +V.sub.2)/S
where V.sub.1 denotes a signal level which is affected by resonance in the nasal cavity and V.sub.2 denotes a signal level which is not affected by resonance in the nasal cavity. Therefore, if V.sub.2 is significantly smaller than V.sub.1, the degree of nasality is approximately given by V.sub.1 /S, i.e., the degree of nasality is nearly in proportion to the nasalized components. However, in the case of a person with a slight or intermediate cleft palate, the condition V.sub.1 .gtoreq.V.sub.2 often develops and the signal level V.sub.2 is more significant. Therefore, the degree of nasality is not necessarily in proportion to the nasalized components. Thus, according to the conventional art, the degree of nasality is not exactly determined in all cases, i.e., the values are not clearly and exactly distinguishable for a person with a normal palate compared with a person with a slightly cleft palate.
According to the above-mentioned conventional method, furthermore, it is difficult to detect only pneumatic vibration which has passed through the nasal cavity by using the vibration detector. The vibration detector detects pneumatic vibration which passes through the vocal tract, rather than the nasal cavity, via the human tissue surrounding the vibration detector. Therefore, it is difficult to extract only nasalized sounds and the results are not exact.